Methanol, solvation numbers

Nakamura S, Meiboom S. Proton magnetic resonance studies of the solvation shell of Mg " " in methanol. Solvation number and exchange rate. J Am Chem Soc. 1967 89 1765-1772. 

The structure of the 1 1 methanol solvate of olanzapine has been reported, where pairs of olanzapine molecules form a centrosymmetric dimer by means of C—H—-7t interactions [66]. The solvent molecule was linked to the drug substance through O—H-N, N—H O, and C—interactions. In a new polymorph of the 1 1 dioxane solvatomorph of (+)-pinoresinol, the structure was stabilized by O—H O hydrogen bonds between the compound and the solvent [67], Two new polymorphs of 2-cyano-3-[4-(/Y,jV-diethylamino)-phenyl]prop-2-enethioamide and its acetonitrile solvatomorph have been characterized [68], Although crystallization of the title compound was conducted out of a number of solvents, only the acetonitrile solvatomorph could be formed. 

Again, solvation numbers in methanol can be calculated by altering the constants to account for halving the number of O-H oscillators per solvent molecule, giving Eqs. (4) and (5) ... 

A method of prediction of the salt effect of vapor-liquid equilibrium relationships in the methanol-ethyl acetate-calcium chloride system at atmospheric pressure is described. From the determined solubilities it is assumed that methanol forms a preferential solvate of CaCl296CH OH. The preferential solvation number was calculated from the observed values of the salt effect in 14 systems, as a result of which the solvation number showed a linear relationship with respect to the concentration of solvent. With the use of the linear relation the salt effect can be determined from the solvation number of pure solvent and the vapor-liquid equilibrium relations obtained without adding a salt. 

The solvation number n remains an adjustable parameter in the comparison of spectroscopic and thermodynamic data. Taking n = 4 for Na+, and n = 8 for Cl (for which there is strong evidence from x-ray scattering investigations (33, 34) and Gourary and Adrian crystal radii (35), (to which half the 0-0 distance in ice and solid methanol (1.38 A) is added for given ro) allows Equation 70 to be used to calculate the electrostatic contribution for each ion. Such at-... 

The preferential solvation formed between salt and solvent molecules causes a salt effect on vapor-liquid equilibria. A method of prediction of salt effect based on the preferential solvation number was reported previously for the case in which salt was solved below the saturation level. The idea introduced in this chapter applies for salt solved in saturation. The alcohol-ester-calcium chloride system for which the preferential solvation was thought to be formed was examined. Specifically, calcium chloride dissolves readily in alcohol but only sparingly in ester. Thus, when calcium chloride is solved into alcohol-ester mixed solvent, the calcium chloride will form a preferential solvation with alcohol only. Methanol-methyl acetate, butanolr-butyl acetate, and methanol-ethyl acetate systems were selected for the mixed-solvent systems. 

Figures 4, 5, and 6 indicate caluculated results of the preferential solvation numbers for the three systems. As shown by each figure, preferential solvation numbers are almost constant against compositions of the solvent. On the other hand, the concentration of salt increases linearly against an increase in the concentration of alcohol in the solvent as indicated in Figures 1, 2, and 3. This fact denotes that for an increase of solvent which forms a preferential solvate in a solvent mixture, the salt required to form a certain solvation number with that solvent is dissolved. For essential concentration x1SL in Equations 3 and 4, which are required in calculating solvation numbers, the data observed by the author et al. (I) were used for the methanol-ethyl acetate system ...

Figure 4. Preferential solvation number in the methanol-ethyl acetate system at 1 atm (O), CaCl2 5 wt % (A), CaCl2 10 wt % (V), CaCl2 20 wt % (D)y CaCl2 25 wt % (%), CaCl2 saturated (1).

With the substrate (Z)-2-benzamido-(3,4-dimethoxy)-dnnamic acid 5d, as used in the technical process, we could realize turnover numbers of 10000-12000 in our laboratory under very careful oxygen exclusion. However, when substrate suspensions were used, this good result could be obtained only with the colorless methanol solvate 5dxMeOH. It is not clear why the methanol-free, yellow colored samples of the substrate with the otherwise same chemical purity led to a much poorer result and with a TON of only 2000. This yellow solvent-free modification can be obtained... 

Perhaps the best method for the determination of the solvation numbers of cations in their non-aqueous solutions is nuclear magnetic resonance (NMR) spectroscopy [Sw 62], The procedure is based on the fact that under optimal conditions the NMR absorption band of the coordinated solvent molecules separates from that of the free solvent, and the ratio of the magnitudes of the areas under the two bands provides direct information on the solvation number of the cation. The conditions for the separation of the two NMR signals are that the rate of exchange between the coordinated solvent and the free solvent molecules should be comparatively low, and the spin relaxation time should be short. Successful studies in dimethyl sulphoxide [Th 66], ditnethylformamide [Fr 67a, Fr 67b, Ma 67] and methanol [A1 69b] have been reported. Jackson et al. [Ja 60] have developed a method that can also be used in cases where the proton resonance signals of the coordinated solvent molecules and the free solvent are so close to one another that they do not separate [Ch 68, Al 69a]. 

It has been shown in recent years that definitive solvation numbers may be obtained for certain metal cations in water, methanol , liquid ammonia , N, N-dimethylformamide 2 and dimethyl sulphoxide by use of NMR-techniques. For the Mg+ in liquid ammonia the unexpected solvation number of 5 has been found . In methanol six solvent molecules were found associated, with each magnesium ion in the primary solvation sphere. Since only one Lorentzian signal was observed for bound ammonia, the exchange between non equivalent ammonias must be rapid. The most important factor in the formation of the pentaammoniated magnesium ion has been assumed to be rather strong ion pairing, which is known to occur in liquid ammonia . 

Richardson and Alger have measured the solvation number and exchange rate for a number of AF and Ga salts in ethanol and methanol, using the OH proton signal from the free and bound alcohol. They obtain solvation numbers ranging from 4 to 7 but point out that the situation is sometimes complicated by the intrusion of the anion (chloride or nitrate) into the inner co-ordination sphere of the metal. They do conclude, however, that in all of the cases they studied the solvent-exchange process is 5n2. 

The results show that there is a marked break between solvation by protic solvents (water, alcohols) and aprotic solvents (MeCN, etc.), and suggest that ammonia and amines resemble aprotic rather than protic solvents. There is a shift to high energy in on going from H2O to MeOH which was interpreted in terms of a higher solvation number for Ij q than Imcoh hence a large cavity for water. I suggest below that such a situation is common for many aqueous versus methanolic solutions. 

Table 3.1 Proton resonance shifts for various salts and individual ions in methanol, together with solvation numbers required to fit the correlation of Figure 3.7...

Figure 3.7 Correlation between infrared shifts Av (cm ) and proton resonance shifts Av (ppm) for solutions of salts in methanol. The origin is the point for monomeric methanol in tetra-chloromethane. The key, together with the solvation number required to fit the correlation, is...

Table 3.2 Solvation numbers for a range of solutes in water and in methanol, estimated from infrared spectroscopic data...

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